UK patent specifications Nos. 1 480 866 and 1 541 767, and German Offenlegungsschrift No. 2 702 784 all relate to processes for the separation of nitrogen from air employing an adsorbent which has the ability to effect a separation as between the two major components of air by virtue of its more rapid adsorption of oxygen than nitrogen. The adsorbent is typically molecular sieve carbon, for example, as described in UK patent specifications No. 1 364 674 and 1 424 501. In operation, a bed of the adsorbent is put through a cycle of operations which includes an adsorption step during which time air is passed through the bed, most of the oxygen and a proportion of the nitrogen and substantially all the carbon dioxide and water vapour in the feed are adsorbed, and a nitrogen-rich product gas is supplied from the outlet of the bed, and a desorption step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure through its inlet and or evacuated through its inlet so that the adsorbed gases are substantially removed from the bed thereby preparing it for the next adsorption step.
In practice, two adsorbent beds are employed and operated on similar cycles which are sequenced to be out of phase with one another by 180.degree. so that when one bed is on its adsorption step, the other bed is on its desorption step, and vice versa. Moreover, it is usual to equalise the pressures in the two beds between each step by connecting the two bed inlets together and connecting the two bed outlets together. With these connections made the gas within the void spaces of the bed which has just completed its adsorption step is sucked into the bed which has just completed its desorption step by virtue of the pressure difference which exists between the beds at that stage, and this is found to be beneficial in maximising the product output because the gas in such void spaces will have already become somewhat enriched in nitrogen.
In commercial practice it is likely that the demand for the product gas may vary or, perhaps, an occasion may arise in which the purity of the product gas may need to be varied. It is an aim of the present invention to provide a process and apparatus which is capable of meeting such need.